To alleviate the membrane fouling in membrane bioreactor, a kind of modified diatomite was used as anti-fouling agent to examine the effect on sludge properties in submerged membrane bioreactor for synthetic domestic wastewater treatment. Three submerged membrane bioreactor setups were operated under fixed membrane flux 21.9m3/m2·h, meanwhile, modified diatomite was added into each membrane bioreactor with dosage of 0mg/L, 1000mg/L and 2000mg/L respectively. Sludge particle size, extracellular polymeric substances and molecular weight distribution were characterized as the activated sludge properties in this study. The experiment results showed that with the increase of modified diatomite dosage, the number of sludge particle size less than 10μm was declined, however, the number between 10–20μm was increased correspondingly. Total extracellular polymeric substances and big molecular weight substances were decreased remarkably with modified diatomite addition dosage of 1000mg/L. The results indicated that addition of modified diatomite could effect of sludge properties in submerged membrane bioreactor. Therefore, membrane filtration performance could be improved by modified diatomite adding which alleviate membrane fouling directly.